1. Field of the Invention
The present invention relates to liquid crystal display devices including a backlight and, more particularly, relates to transmitted-light-display type liquid crystal display devices including LEDs (Light Emitting Diodes) as a light source.
2. Description of the Background Art
FIG. 8 is a block diagram illustrating an LED light source stabilization/control circuit 90 disclosed in Armand Perduijn et al., “43.2: Light Output Feedback Solution for RGB LED Backlight Applications”, “SID2003 CD-ROM”.
The stabilization/control circuit 90 illustrated in FIG. 8 is broadly divided into a color control means 22, a brightness control means 23 and an LED driving duty ratio control means 24.
The color control means 22 is structured to include an addition means 222, an integration means 223, a PWM control block 224, an LED driving/operation detection block 225 and a low-pass filter 226.
The brightness control means 23 is structured to include a brightness sensor 231, an adder 232 and a brightness feedback circuit 233, and the LED driving duty ratio control means 24 is structured to include an adder 241, a maximum duty ratio setting means 242 and an LED driving duty ratio clipping circuit 243.
The brightness control means 23 and the LED driving duty ratio control means 24 constitute a brightness adjustment means 26, in cooperation with a brightness setting means 25 for setting a brightness value (Y′).
In the stabilization/control circuit 90, XYZ values (a color set value) as control targets are set by a color setting means 20, and these values and the output of the brightness adjustment means 26 are supplied to a multiplication means 21 which performs multiplication thereof. The result of the multiplication is supplied to the addition means 222 in the color control means 22.
In addition thereto, the output of the LED driving/operation detection block 225 is fed back to the addition means 222 through the low-pass filter 226 so that the difference between the output from the LED driving/operation detection block 225 and the result of the multiplication from the multiplication means 21 is supplied to the integration means 223.
Further, the output of the integration means 223 is supplied to the PWM control block 224 which calculates the duty ratios for PWM driving of the red, green and blue LEDs. The PWM control block 224 is structured to enable setting the gain of the integration component of the PWM control for the amount of feedback.
The LED driving/operation detection block 225 includes three types of LEDs for generating red, green and blue lights, a PWM driving circuit which individually drives the three types of LEDs, and a color detection means which disperses white light generated from a light guide plate through color filters which are approximated to a CIE1931XTZ color matching function and detects the X′, Y′ and Z′ values (color detected values) of separated lights, wherein light guide plate mixes the red, green and blue monochromatic lights generated from the LEDs into white light.
The output of the PWM control block 224 is supplied to a PWM driving circuit in the LED driving/operation detection block 25.
The X′, Y′ and Z′ values (color detected values) which are output from the LED driving/operation detection block 25 through the low pass filter 226 are also supplied to the brightness control means 23, and a brightness sensor 231 detects only the brightness value Y′ and supplies it to the addition means 232.
On the other hand, a brightness value Y′ set by the brightness setting means 25 and the brightness value Y′ output from the LED driving/operation detection block 25 are supplied to the addition means 232 which outputs the difference therebetween. The difference is supplied to the brightness feedback circuit 233 in the brightness control means 23 where it is subjected to a PID (Proportional, Integral, Differential) comparison control. Further, the brightness feedback circuit 233 is structured to enable setting the gain of the integration component of the PID comparison control for the amount of feedback.
The value resulted from the comparison controlling process by the brightness feedback circuit 233 is supplied to the addition means 241 of the LED driving duty ratio control means 24 which outputs the difference between this value and the output from the PWM control block 224 to the LED driving duty ratio clipping circuit 243.
On receiving the output of the addition means 241, the LED driving duty ratio clipping circuit 243 calculates a PWM duty ratio (common to red, green and blue colors) for the LEDs, on the basis of the output, Then, the result of the calculation is supplied to one of the inputs of the multiplication means 21.
Further, the LED driving duty ratio clipping circuit 243 is structured to enable setting the gains of the proportional component and the integration component of the PID comparison control for the amount of feedback.
In the aforementioned stabilization/control circuit 90, when the duty ratio of the PWM driving for the LEDs reaches a certain value, the overall gain is reduced and a feedback operation is performed in such a manner as to prevent the color fluctuation due to the clipping of the duty, thereby stably controlling the light intensities of the red, green and blue LEDs of the backlight source and the balance thereamong.
FIGS. 9A, 9B and 9C illustrate exemplary temperature-induced fluctuation characteristics of the light emission spectra of blue, green and red LEDs.
In FIGS. 9A, 9B and 9C, the horizontal axis represents the wavelength while the vertical axis represents the light intensity (relative value), and there are illustrated, in a superimposing manner, the light-emission spectra of the LEDs of the respective colors, for casing temperatures Tc of +25° C., +85° C. and −20° C. at the casing housing the LEDs.
Further, in FIGS. 9A, 9B and 9C, the light-emission spectra at the respective temperatures are illustrated, on the assumption that the peak light intensity (λ peak) at a casing temperature of +25° C. is 1.
As can be seen from FIGS. 9A, 9B and 9C, the light-emission intensities of the LEDs of the respective colors are varied with the temperature. Conventionally, such effects of temperature changes have been compensated for through feedback controls using, for example, the stabilization/control circuit 90 described with reference to FIG. 8.
Further, Japanese Patent Application Laid-Open No. 2002-311413 (FIG. 4) discloses a technique for determining the brightness of a backlight and the temperature within the device and then correcting the brightness on the basis of the temperature within the device in order to attain a target brightness.
As described above, the stabilization/control circuit for the LED light source described in the aforementioned literature can stably control the brightness and the chromaticity of only the backlight source. However, the light sensing circuit used as light detection means has the possibility of causing fluctuations of the electric current output of the photodiodes used for light detection due to temperature changes and also has the possibility of causing fluctuations of the resistance of the resistor used in the amplification circuit for converting the electric current outputs of the photodiodes into voltages, due to temperature changes.
FIG. 10 illustrates the relationship between the output voltages of light sensors for red, green and blue colors and the operating temperature.
In FIG. 10, the horizontal axis represents the temperature (° C.) while the vertical axis represents the output voltage (V), wherein the output voltage characteristic of the light sensor for the red color (R) is plotted with a rectangular mark, the output voltage characteristic of the light sensor for the green color (G) is plotted with a round mark, and the output voltage characteristic of the light sensor for the blue color (B) is plotted with a triangular mark. The left vertical axis and the right vertical axis have different scales and the left vertical axis is marked in 0.005 V increments while the right vertical axis is marked in 0.2 V increments. The left vertical axis represents the output voltage of the light sensor for the green color, while the right vertical axis represents the output voltages of the light sensors for the red and blue colors.
In spite of the scale difference, FIG. 10 shows that the output voltage of the light sensor for the green color exhibits greatest temperature dependence, and there are also observed slight fluctuations in the output voltages of the blue and red light sensors.
Further, a liquid crystal display panel employing LED light sources as the backlight also exhibits a spectral transmittance which varies with the temperature.
FIG. 11 illustrates a temperature characteristic of the transmittance of a liquid crystal display panel.
In FIG. 11, the horizontal axis represents the wavelength (nm) while the vertical axis represents the light intensity (relative value) which is transmitted through the liquid crystal display panel, wherein there are illustrated the transmittances for respective wavelengths for temperatures of 24.5° C. and 43° C. at the liquid crystal display panel, thereby showing that the transmittances are decreased with increasing temperature.
In FIG. 11, the transmittances for respective wavelengths are illustrated, on the assumption that the light intensity for a wavelength of 523 nm at a liquid crystal display panel temperature of 24.5° C. is 1.
Since the operating temperature of the light sensors and the operating temperature of the liquid crystal display panel are increased with the elapsed time after power-on, the detection characteristic of the light sensors and the spectral transmittance of the liquid crystal panel are also changed with the elapsed time.
FIG. 12 illustrates the result of tests for the fluctuation in a light feedback controlling operation for an experimentally-produced liquid crystal display panel including a stabilization/control circuit equivalent to the stabilization/control circuit 90 illustrated in FIG. 8, in the cases of using a cabinet (enclosure) or no cabinet.
In FIG. 12, the vertical axis represents the color difference (ΔEab) from finally stably obtained brightness and chromaticity, while the horizontal axis represents the elapsed time (min).
As can be seen from FIG. 12, the liquid crystal display requires about 250 minutes for stabilizing the color difference when it employs the cabinet while it can stabilize it within about 100 minutes when it employs no cabinet. Thus, the feedback converging time is largely varied depending on whether or not there is a cabinet.
It can be considered that the aforementioned phenomenon is caused by the difference in the heat release at the backlight LED light source portion between when there is a cabinet and when there is no cabinet.
As described above, conventional stabilization/control circuits for LED light sources have been susceptible to the temperature change within the cabinet of the liquid crystal display panel and the temperature change in the liquid crystal display panel, thereby requiring longer times for stabilizing the brightness and the chromaticity.
Further, although Japanese Patent Application Laid-Open No. 2002-311413 discloses correction of brightness on the basis of the temperature in the device in order to attain a target temperature, the device does not employ LEDs as the light source.